Container for cooking food therein

ABSTRACT

A container is provided consisting of a polyethylene terephthalate tray with a polyethylene terephthalate lid bonded, by a novel adhesive, to an upper surface of the tray. The adhesive is coated only onto this upper surface, which surface is in the form of a peripheral flange. The adhesive is free from flow at cooking temperatures of up to about 400*F. and the bond strength at these cooking temperatures is sufficient to prevent leakage from the tray, yet, nonetheless, permit peeling of the lid from the tray without tearing, after cooking.

United States Patent [191 Kane 1 1 CONTAINER FOR COOKING FOOD THEREIN[75] Inventor: William P. Kane, Bon Air, Va.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: Nov. 10, 1972 [211 Appl. No.: 305,250

[52} US. Cl 229/43, 156/69, 156/334, 229/35 R, 229/48 T, 426/106,426/396,

[51] Int. Cl. A47j 27/00, B65b 7/28, 865d 1/O0, B65d 5/64, 1365b 29/08[58] Field of Search ..426/106,113,l14,123. 426/396, 410, 412, 413, 415;206/46 F;

T; 229/35 R, 43, 48 T [56] References Cited UNITED STATES PATENTS2,650,213 8/1953 Hofrichter, Jrv 161/231 X 2,834,686 5/1958 Reuman426/415 X 1 1 Feb, 11,1975

3,189,573 6/1965 Oken 260/28.5 AV 3,192,091 6/1965 Hey et a1. 156/693,215,657 11/1965 Beresniewkz et a1. 260/28.5 AV 3,414,647 12/1968Saunders et a1. 264/235 3,455,720 7/1969 Davies et a1 260/75 T 3,496,1432/1970 Sig'gel et a1 260/75 T 3,505,259 4/1970 Hallis, Jr. 260/28.5 AV3,719,625 3/1973 Parker 260/28.5 AV

Primary Examiner-Frank W. Lutter Assistant ExaminerSteven L. Weinstein[57] ABSTRACT A container is provided consisting of a polyethyleneterephthalate tray with a polyethylene terephthalate lid bonded, by anovel adhesive, to an upper surface of the tray. The adhesive is coatedonly onto this upper surface, which surface is in the form of aperipheral flange. The adhesive is free from flow at cookingtemperatures of up to about 400F. and the bond strength at these cookingtemperatures is sufficient to prevent leakage from the tray, yet,nonetheless, permit peeling of the lid from the tray without tearing,after cooking.

3 Claims, 6 Drawing Figures PATENTED 1 $865,302

SHEEI 1 OF 2 PATENTEU 1 H975 3,885,302

SHEET 2 OF 2 SURFACE FREE ENERGY (DYNES) I 50 I00 ETHYLENE GUPOLYNERFIG-5 SURFACE FREE ENERGY (UYNES) RELATIVE PEEL STRENGTH (ARBITRARYUNITS) CONTAINER FOR COOKING FOOD TI-IEREIN BACKGROUND OF THEINVENTION 1. Field of the Invention This invention is a container'consisting ofa tray having a lid secured to a flange of the tray by anovel adhesive.

This invention further is a method of using this novel container, boththe lid and tray of which are polyethylene terephthalate, with anadherent bond while foods are heated at temperatures up to about 400F.and, after cooking and cooling, the lid of which may be peeled with easefrom the tray to permit access to the cooked contents therein.

2. Description of the Prior Art US. Pat. No. 3,547,388 to Hernnes isexemplary of the prior art. Such patent discloses a food packageincluding a one piece container and a two-layer laminated film coverjoined to a flange thereof. The upper layer of the cover film laminatehas properties similar to the material from which the container body ismade while the lower layer of the film cover laminate permits bonding,but easy separation from the container body.

Other patents of the prior art of interest to the container and methodof using such container (including cooking in it and opening of it,after cooking) also of this invention include Pat. Nos.: 3,547,338;3,491,935; 3,616,898; 3,202,271; 3,615,707; 3,655,503; 3,656,210;3,530,917; 3,561,668.; and 3,498,525.

None of these patents show the thermally stable container of the instantinvention in which only the upper surface of the flange of the tray iscoated with an adhesivc, to minimize the exposure of the adhesive to thecontents in the tray, and in which food may be cooked or reheated atnormal cooking temperatures up to about 400 F. and the lid of which maybe peeled easily from the tray, after cooking.

SUMMARY OF THE INVENTION According to the present invention, containersare provided for the packaging of food and for use in the subsequentcooking and reheating of the food in preparation for human consumption.

The invention comprises a versatile container consisting of a thermallystable tray and a lid secured to a flange of the tray by an adhesive onthe flange which provides a secure seal at heating temperatures and aseal peelable after heating and cooling to room temperature.

Trays of various materials have been widely employed for packaging anddirect consumption therefrom of preprocessed foods. Limitations ofmaterials from which trays were formed as well as limitations in othercomponents of the package have resulted in restrictions in use of suchpackages.

Previously it has been the practice to form trays of heat scalablepolymers, such as vinyls and oleflns, to facilitate securing lidsthereon, but these in general do not possess the thermal stabilityrequired for reheating foods. Formed trays of aluminum foil have beenemployed, but these lack versatility in that they are unsuitable for usein microwave ovens.

The requirements for a satisfactory, versatile tray to which the presentinvention is directed are that it should have a closure, i.e., a lid,that is bonded to the tray body so that the bond does not rupture duringreheating the container and its contents up to about 400F., and yet withthe sealed lid readily peelable to gain access to the contents aftersuchheating and cooling to temperatures suitable for direct consumption,i.e., to F. Further, the combination should have thermal stability inthat the tray or lid should not embrittle or be prone to cracking uponheating. Likewise, it should not become distorted upon heating. Finally, the lid should be adhered in a manner to minimize exposure ofthe contents by the adhesive.

The present invention provides a tray and lid combination capable ofbeing sealed with an adhesive critically located and having theseparticular characteristics.

The container is formed from a tray of polyethylene terephthalate towhich is secured or bonded a polyethylene terephthalate lid. Thisbonding of the lid to the tray is accomplished by use of a noveladhesive that is coated only onto a flange of the tray. By doing this,undue exposure of the adhesive to the contents of the tray is precludedor prevented. The food in the tray may be reheated or heated totemperatures up to about 400F. without having any problems as far as thebond is concerned; it remains a tight seal. After cooking, this bond iseasily broken when the lid is peeled from the tray.

In short, then, this invention is a container in the form of a liddedtray that can be effectively used for cooking, or reheating at oventemperatures and which after cooking and cooling may be easily opened,by peeling the lid from the tray to reach the food contained therein,without damage to the tray or contents.

More specifically, these requirements are provided by a containercomprising a tray of polyethylene terephthalate and a lid ofpolyethylene terephthalate bonded thereto with an adhesive applied onlyto an upper surface of the tray, the adhesive being free from flow atcooking temperatures up to about 400F. and having a bond strength atsuch cooking temperatures sufficient to prevent leakage from the trayand the lid being peelable from the tray without tearing after heatingat such cooking temperatures and cooling to room temperatures. Thecontainer as above described wherein the bond strength is greater than100 gramslinch at 400F. and less than 500 grams/inch at roomtemperature, and also wherein the polyethylene terephthalate in the trayhas an intrinsic viscosity of at least 0.65 in a 60/40 mixture oftetrachloroethane/phenol at 25C also provides these requirements.

The invention also comprehends a container comprising: a dish-like trayof polyethylene terephthalate having an intrinsic viscosity of at least0.65 in 60/40 tetrachloroethane/phenol, the tray having abottom part anda body part and an outwardly projecting peripheral flange having anupper surface on the upper extremity of the body part; an adhesive onlyon the upper surface ofthe flange, the adhesive having a bond strengthat least 100 g./in. at 400F. and less than 500 g./in. at 70F, a meltflow of less than 0.2 grams at 320F. (C) for 10 minutes and a surfacefree energy of less than 30 dynes; and, a substantially flat coverhaving its lower surface substantially coextensive with and bonded tothe flange by the adhesive.

Also comprehended in the foregoing structures is the aforesaidpolyethylene terephthalate containing up to 1.0 percent talc and up to 2percent TiO The specific combination of adhesive used in sealing the lidto the tray, the tray and the location of the adhesive of the presentinvention permits these strong bonds at higher cooking temperatureswhich are, nonetheless, easily peelable after cooking, and do notcontaminate the contents. The adhesive is coated only onto the flange ofthe tray, thus sealing of the lid is simplified, and there is a minimumof contact potential with the contents. For example, if the adhesive iscoated onto only selected parts of the lid, registration problems may bepresented in making certain that none of it is applied so as to overliethe food when the lid is placed on the tray.

To gain access to the contents of the container the lid is peeled fromthe upper margin of the container thereby exposing the containercontents.

In containers in which components are hermetically sealed to oneanother, there have been several approaches in facilitating the openingof the container and thus exposing the container contents. For example,the film covering or lid has been made from a different plastic materialthan that from which the container is made. By selecting two suchdifferent plastic materials, a weld or bond can be obtained between thefilm covering and container body which is also properly hermeticallysealed, and further permits easy removal of the film covering from thecontainer body.

Such prior art containers have the disadvantage, however, that thecontainer body or covering film, in most cases the covering film,consists of a plastic material having a low softening range or meltingpoint, and as such they are not well suited for the packaging of foodsubstances which are to be externally heated, such as in a hot airchamber, whether it be for the purpose of sterilization or whether it befor the purpose of heating the contents of the package prior toconsuming the same. It has generally been the experience with such priorart containers that the part which is formed or made from the plastichaving the low temperature stability softens during heating and deforms,thus making the container potentially undesirable from a sanitary, as aserving tray for edible materials, and/or aesthetic viewpoint.

Accordingly, the instant invention provides a novel container whichexhibits unique packaging versatility and which overcomes disadvantages,such as loss of bondng strength at high cooking temperatures or lack ofease of opening after cooking or which minimizes the exposure of thecontents to bonding adhesives, of currently used containers. In itsbroadest terms, this invention is a disposable container which comprisesa lid of polyethylene terephthalate bonded to a tray of the samematerial by a novel adhesive critically positioned only on a flange ofthe tray. This container, and method of using it, will now be describedin greater detail.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a side elevational view, withparts broken away for clarity, ofa container of this inventionconsisting of a tray having a lid secured to an upper flange thereof bya novel adhesive.

FIG. 2 is an enlarged cross-sectional view of parts of the container ofFIG. 1.

FIG. 3 is a perspective, exploded, view of the container of FIG. 1showing the adhesive only on the flange of the tray ready to bond thelid thereto.

FIGS. 4 and 5 are graphs showing relationships of surface free energy(in dynes) to the ethylene copolymer component of the adhesive and therelative peel strength to this surface free energy.

FIG. 6 is a schematic illustration of an apparatus for applying anadhesive to a flange of a tray,

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is a container ofa thermoplastic material (polyethylene terephthalate) which may betightly sealed and which, nevertheless, may be opened easily.

In recent years various products of perishable and durable nature havebeen packaged in containers formed from plastic materials andparticularly thermoplastic materials which can be heat sealed. One typeof container employed for packaging such items, such as frozen foods andthe like, is a tray container which can be made from heat-scalablethermoplastic materials such as polyethylene, polypropylene, polyvinylchloride and the like. The tray is provided with a lid formed of aheat-scalable thermoplastic material generally the same material as inthe tray itself. Sealing of the container is conveniently accomplishedby fusing the lid and tray together by means of heat and pressure afterthe contents of the container are placed therewithin. This type ofcontainer, after being properly heatsealed, is substantially imperviousto vapors, moisture and satisfactorily protects the contents thereofagainst contamination from external sources. These containers can beproduced relatively inexpensively in a variety of shapes and sizes butgenerally are unsuitable for certain uses when being subjected totemperatures of up to 400F. whereby the contents can be heated andcooked or sterilized as is sometimes desired when certain food items arepackaged.

With the thermoplastic materials used to form the containers, it isextremely difficult to peel the lid from the tray when attempting toremove the cover or lid from the tray to reach the contents. Moreover,with a tight bond or fusion between the lid and the tray it is virtuallyimpossible to initiate a tear therebetween. The difficulties in openingthese containers are further increased when the containers are subjectedto heat as when the contents are heated while in the container.

The container of this invention solves these and other problems.

Referring to the drawings and FIG. I in particular, there is shown atypical novel container C of the present invention.

Such container C is formed of polyethylene terephthalate and consists ofa tray 10 having a lidll secured to it by a novel adhesive 13.

The tray 10 includes a bottom part 14 with a body part or sides 15connected thereto and further includes a peripheral flange 16 having anupper surface 17 on the upper extremity of the body part 15.

With reference to the drawings, the tray 10 is shown to be dish-like inshape but can be of any desired shape such as circular, square,triangular and so forth. Tray 10 is formed by conventional operationsfrom a thermoplastic material, polyethylene terephthalate.

The lid 11 is made of the same thermoplastic material (polyethyleneterephthalate) as the tray 10 may be provided with a pull-tab whichpreferably extends beyond the marginal edge of the associated flange 14.One or more such pull-tabs can be provided for peeling the lid from thetray.

After placing the contents in the tray 10, the cover or lid 11 is placedthereover with its peripheral edges substantially coincident with thoseof flanges 16. The cover 11 is sealed to flange 16 by the adhesive 13 bymeans of heat and pressure in a conventional manner to form a sealextending the entire length of the flange 16. The line of heat seal isdisposed in parallel relationship to the annular surface and toward theperipheral edge of the flange.

In the embodiment illustrated in the drawing. the container C includesthe dish-like tray having the bottom part 14 and body part 15 which iscircumferentially continuous and which extends upwardly (and outwardlyif nesting between containers is desired) from the bottom and terminatesin the radially outwardly directed circumferentially continuous flange16 at the open upper end or upper margin of the container. The containerfurther includes the covering film or lid 11 bonded to the radiallyoutwardly directed flange of the container body.

The container body or tray 10 is thermoformed into a one-piece shape asillustrated from a plastic material having a predetermined strength andrigidity at elevated temperatures. A material which can be subjected toelevated temperatures in a hot air oven is polyethylene terephthalatewhich does not distort easily at elevated temperatures and also retainsits strength and rigidity at such temperatures. The lid is also madefrom a plastic material having properties similar to the container bodyor tray such as polyethylene terephthalate. The lid so made permitsbonding to the radially outwardly directed flange of the tray, butpermits easy separation therefrom.

The container body or tray 10 is thermoformed in a one-piece shape, asillustrated, by conventional forming apparatus, in particular apparatushaving a heated mold and ram to enhance crystallization of the formedstructure. Starting with unoriented, amorphous polyethyleneterephthalate film (lV=0.65 or greater in 60/40 TCE/phenol) with anominal thickness of 15 mils (0.015 inch) containing 0.5 to L5 percenttitanium dioxide (TiO and 0.25 to 1 percent talc, the former as anopacifier and the latter to enhance the rate of crystallization, thefilm is preheated from about 80C. (176F.) to about 150C. (302F.),preferably l00-l 10C. for not more than 30 seconds. (Preheating shouldnot exceed about 30 seconds or crystallization may occur, making formingimpossible.) The mold and the ram, (preferably a ram is employed) isheated to l30-l85C. (266465F.). preferably l50-l55C'. The residence timein the mold may be up to 30 seconds; however, with the upper range oftalc, residence time as low as 6-7 seconds are sufficient to form andcrystallize the tray, and are preferred. Crystallization may continuemomentarily after the tray is removed from the mold.

Due to the ready separability and bonding characteristics of the lid 11relative to the radially outwardly directed flange 16 of the tray 10 dueto the adhesive 13, the lid 11 may be easily stripped or peeled from thetray 10, through the use of suitable finger tabs or the like (notshown), as a result of the bond strength which is provided between thelid and the flange ofthe tray. The high temperature stabilitycharacteristics of the tray 10 as well as lid, provides an overallcontainer which can be subjected to elevated temperatures as in a hotair oven without incurring any of the disadvantages previously mentioneddue to this bond.

An adhesive of this invention may be formed from a compositionconsisting essentially of (a) from 60 to percent by weight of anethylene copolymer" of the type defined and disclosed in U.S. Pat. No.3,2l5,657 to Beresniewicz et al. and (b) from 40 to 30 percent of apetroleum wax.

Minor amounts of other materials such as pigments, dyes, anti-oxidantsand the like may also be present, but, in general. the ethylenecopolymer and petroleum wax should constitute at least percent of thetotal composition weight.

The critical aspect of this is the percentage of ethylene copolymer andparaffin wax or petroleum wax in the composition. If the wax content isby weight less than 30 percent and more than 40 percent. marginallyeffective seals, particularly as to peelability, are produced by theadhesive between the polyethylene terephthalate lid and tray afterheating for 1 hour at 400C, for example. The key to acceptability is acombination of melt flow at l60C./l0 minute period in an oven, surfacefree energy of less than 28 dynes. The flow is 0 grams to 0.2 grams at160C. for l0 minutes (ASTM 1238).

An important consideration, as previously mentioned, is the behavior ofthe polymer from which the trays are formed. In the forming process thebody of the tray is oriented and crystallized, and as such is not subject to cracking and exhibits a minimum of dimensional change in theheating-cooling sequence incidental to food preparation or instrumentsterilization. The flange, on the other hand, receives slight, if anyorientation in the forming process, but crystallizes. Accordingly, ifthe intrinsic viscosity of the polyethylene terephthalate in thestructure is of the range customarily used for films and fibers(0.55-0.60 in 60 parts TCE/40 parts phenol), the unoriented crystallizedflange can become brittle and crack, causing leakage and other failureof the bond. The polyethylene terephthalate in the trays, it has beenfound, with an intrinsic viscosity of at least 0.65, is free from suchembrittlement. As is known to those skilled in the art, the polymer fromwhich the film for the trays is made, should have a viscosity slightlyhigher than this level to compensate for losses by depolymerization inprocessing. The extent of such loss will, of course, depend upon processconditions, such as the moisture content of the starting polymer.

The adhesives effective in the present invention are those which bondthe polyethylene terephthalate without special treatment of thissubstrate. Particularly useful are wax-ethylene terpolymer compositionsdescribed in U.S. Pat. No. 3,2l5,657. The terpolymer component providesthe effective bonding agent and the wax acts as a diluent to control thepeel strength and the physical properties (e.g., melt index) of thecomposition.

To determine effective compositions an acid terpolymer resin sold underthe trademark Elvax 4355 an interpolymer of ethylene, vinyl acetate andan organic acid was used at various ratios in a composition with aparaffin wax having a C. melting point. The undiluted terpolymer had amelt index of 6.0 g./l0 min. (ASTM 1238), typically 25 percent vinylacetate and an acid number of 6. The melt flow (index) is a function ofthe percentage terpolymer.

Effective adhesives may be found by determination of the surface freeenergy, a function of the contact angle of the adhesive on thepolyethylene terephthalate substrate. FIG. 4 illustrates this surfacefree energy, as a function of composition. Peel strength can becorrelated with surface free energy, as shown in FIG. 5. lnspection ofthese two figures shows that at about 75 percent Elvax 4355 the rapidrise in surface free energy, with the corresponding unwanted increase inpeel strength, suggests that a surface free energy of about 30 dynes isan upper limit.

The adhesive is applied to the rim of the tray, rather than the lid forsignificant reasons. First, due to health standards in trays intendedfor foodstuffs, as most of them are, it is essential to present theminimum possible Contact between the adhesive and the contents of thetray. Accordingly, with a lid free of adhesive except where bonded tothe tray at the flange, the only contact of the contents and theadhesive in lidded tray of this. invention is at the verythin edge ofthe seal. Applying.

the adhesive on the lid, even if only annularly at the edge of the lid,would require a relatively broad band of the adhesive material to assurecontact with the flange, and would present considerable mechanicaldifficulty and expensive machinery for registering the adhesive with theflange on the tray. 1

Similarly, lids with an adhesive laminate or fully coated with adhesiveto minimize the problem of registration, could present an intolerableexposure of the adhesive to the contents. This could cause difficultynot only from the possible contamination of the contents with toxicmaterials, but also may cause adhesion of the contents, e.g., surgicalinstruments subsequent to heating, as for sterilization.

Apparatus for application of the adhesive to the rim of the tray isschematically illustrated in FIG. 6. Conveyor 20 driven by rolls 21 and22 engages trays l0, either singly or in groups of two or more and movesthem in kiss contact with applicator roll 23. Adhesive, as described, isapplied to roll 23 by hopper-25 with the coating thickness controlled bydoctor blade 26. The adhesive may be applied as a melt, in which caseroll 23 is heated to the softening temperature of the adhesive, or quiteeffectively, it may be applied from a toluene solution, in which caseapplicator roll 23 is at ambient temperatures. With the solvent appliedadhesive it is desirable to employ a heated drying tunnel to remove thesolvent from the adhesive.

Specifically, the instant invention in one embodiment provides acontainer for baking and cooking which comprises:

a substantially flat bottom part;

at least four upstanding sides connected to the flat bottom part; I

the container being formed of substantially amorphous polyethyleneterephthalate; and

a lid of polyethylene terephthalate secured thereto.

In another embodiment, this invention is a container for baking andcooking which comprises:

a dish-like tray of polyethylene terephthalate, the

tray having a bottom part and a body part; an outwardly projectingperipheral flange having an upper surface on the upper extremity, of thebody part;

a substantially flat cover part having lowersurface parts substantiallycoextensive with the flange and in abutting contact-therewith; I

a thin coating of a heat-activated adhesive only on the upper surface ofthe flange;

the adhesive having a melt flow of less than 0.2 g. in

10 min. at 160C. and'a surface free energy of less than 30 dynes. g

And, a method is provided for cooking in these novel containers, whichcooking method includes the steps of: forming a container formed from atray and lid both of polyethylene terephthalate and bonded thereto by anadhesive by the methods previously described; placing food in thecontainer; and, cooking the food at a temperature of up to about 400F.

This invention further is a method of sealing a polyethyleneterephthalate lid to the flange of a polyethylene terephthalate tray toprovide'a critical bond between the lid and tray. This bond or seal issuch that during cooking at temperatures up toat least 400F. thecontainer remains hermetically sealed and, after cooking, the lid may beeasily peeled from the tray. The traymay be suitably formed by heatingthe material from which the tray is formed (i.e., the container or trayblank) and by pressing the heated material to form such'tray with itsbottom part and body part with the flange connected thereto. Thespecific type of polyethylene terephthalate material used in the tray iscritical.

ln forming the tray, the portions of it except the flange may beoriented. Due to lack of orientation the flange, unless it'has specificproperties, tends to crack. Such properties for the polyethyleneterephthalate material which prevents this cracking include an intrinsicviscosity of at least 0.65.

The intrinsic viscosity of the polyethylene terephthalate of the tray ofthis invention is critical to the practice of such invention. lt hasbeen discovered that such intrinsic viscosity must be above 0.65, forreasons further to be explained.

The intrinsic viscosity of polyethylene terephthalate of the presentinvention is determined in a trifluoroacetic acid-methylene chloridesolvent system, since dissolution times are prohibitively long in thetetrachloroethane-phenol solvent system used for conventionalpolyethylene terephthalate. For this determination the relativeviscosity (nr) of the present polymer is determined in a 1 percentsolution at 30C. in a solvent comprising 25 parts by weighttrifluoroacetic acid (TFA) and parts by weight methylene chloride (Ch ClThe relative viscosity in this solvent (relative viscosity is the flowtime of the solution through a capillary viscometer divided by the flowtime'of the solvent) is converted to the relative viscosity which wouldbe obtained in a conventional solvent of 0.6 parts by weight1,1,2,2-tetrachloroethane (TCE) and 1 part by weight phenol at 25C.Using the empirical equation nr(TCE-phenol) 1.0324 nr (TFA-CH Cl 0.19,the relative viscosity in TCE-phenol is calculated. From this relativeviscosity the intrinsic viscosity is determined from the experimentallyconstructed table:

Continued INTRINSIC VS. RELATIVE VISCOSITY POLYETHYLENE TEREPHTHALATE INTCE/PHENOL (This table is extrapolated for relative viscosities above2.42. intrinsic greater than I .00).

Alternatively, intrinsic viscosity can be determined in a solution ofpolyethylene terephthalate in a trifluoroacetic-acid-methylene chloridesolvent and converted to the value in tetrachIoroethylene-phenol by anempirical equation obtained from multi-point viscosity determinations offilm of the priorart and film of the present invention:

in which [17] =the intrinsic viscosity of polyethylene terephthalate ina solvent comprising 40 parts by weight tetrachloroethane and 60 partsby weight phenol; lnh ==the intrinsic viscosity of polyethyleneterephthalate in a solvent comprising 25 parts by volume trifluoroaceticacid, 75 parts by volume methylene chloride.

The polyethylene terephthalate material of this invention is formed froma polymer formed by the condensation reaction of ethyl glycol andterephthalic acid. Particulars of this material are shown and describedin US. Pat. No. 2,465,319 to Whinfield and Dickson. More specifically,polyethylene terephthalate useful in preparing the structure of thisinvention includes (a) polymers wherein at least about 97 percent of thepolymer contains the repeating ethylene terephthalate units of theformula:

o -cn,cmo (LGJL with the remainder being minor amounts of esterformingcomponents.

The polyethylene terephthalate material of this invention also includesblends of polyethylene terephthalate with a different polymer compatibletherewith wherein the blend contains at least 70 percent polyethyleneterephthalate, as defined above.

This material does not get brittle with age, it has a long shelf life;it provides a permanent, nonyellowing, dimensionally stable base and hasexcellent resistance to grease, oxygen and moisture. Further, it isresistant to initial tear and scratching and can withstand heavy impactand prolonged flexing. These and other properties are of great value inpackaging applications.

This invention, then, is a container comprising a tray of polyethyleneterephthalate and a lid of polyethylene terephthalate bonded theretowith an adhesive applied only to an upper surface of the tray, theadhesive being free from flow at cooking temperatures up to about 400F.and having a bond strength at such cooking temperatures sufficient toprevent leakage from the tray and the lid being peelable from the traywithout tearing after cooking at such cooking temperatures.

The bond strength is greater than 100 grams/inch at 400F. and less than500 grams/inch at room temperature.

The container of this invention comprises a dish-like tray ofpolyethylene terephthalate. the tray having a bottom part and a bodypart and an outwardly projecting peripheral flange having an uppersurface on the upper extremity of the body part; an adhesive only on theupper surface of the flange. the adhesive having a melt flow of lessthan 0.2 grams at 320F. (160C) for 10 minutes and a surface free energyof less than 30 dynes; and, a substantially flat cover part having lowersurface parts substantially coextensive with and bonded to the flange bythe adhesive.

A method of making this container includes the steps of: forming a trayhaving a bottom part and a body part with a flange having an uppersurface connected to such body part, the tray being of polyethyleneterephthalate; coating an adhesive onto the upper surface only of theflange of the tray; placing a lid of polyethylene terephthalate havinglower outer surface parts in abutting contact with the adhesive andsealing the abutting surface portions of the lid and tray together.

The adhesive is a composition consisting essentially of (a) 30 percentto 40 percent by weight of a petroleum wax and (b) percent to 60 percentby weight of an ethylene copolymer.

The wax and ethylene copolymer together constitute at least percent ofthe total weight of such composition.

The ethylene copolymer is of an acid terpolymer resin consisting of aninterpolymer of ethylene, vinyl acetate, and an organic acid.

The adhesive is a composition consisting essentially of (a) at least 30percent by weight of a petroleum wax and (b) an ethylene copolymer andwherein such wax and ethylene copolymer constitute at least 80 percentof the total weight of the composition.

This invention further is a method including the steps of: placing foodin a tray formed of polyethylene terephthalate; coating an adhesive onlyonto an upper surface of such tray; bonding a lid formed of polyethyleneterephthalate to the upper surface of such tray by means of suchadhesive; cooking the food contained in the tray at a temperature of upto about 400F.; and peeling the lid from the tray after cooking.

The bond between the tray and lid is hermetically tight when cooking thefood contained in the tray at a temperature of up to about 400F. andwherein the bond between the tray and lid is of peelable strength aftercooking at up to about such 400F. temperature and at room temperaturewhereby the lid may be peeled from the tray without tearing.

I claim:

1. A container for cooking food including:

the combination of a dish-like tray of polyethylene terephthalate, thetray having a bottom part and a body part and an outwardly projectingperipheral flange having an upper surface on the upper extremity of thebody part;

an adhesive of an ethylene copolymer and petroleum wax having acomposition consisting essentially of (a) 30 percent to 40 percent byweight of petroleum wax and (b) 70 percent to 60 percent by weight ofsuch ethylene copolymer, positioned only on the upper surface of theflange. the adhesive having a melt flow of less than 0.2 grams at 320F.(160C) for 10 minutes and a surface free energy of less than 30 dynes;and

substantially flat cover part of polyethylene terephthalate having lowersurface parts adapted to be bonded to the flange by the adhesive,wherein said adhesive is free from flow at cooking temperatures up toabout 400F and said combination exhibits a 12 bond strength greater than100 grams/inch at 400Fand less than 500 grams/inch at room temperaturewhen said cover part is bonded to said flange.

2. The container of claim 1 wherein the polyethylene terephthalate inthe tray has an intrinsic viscosity of at least 0.7.

3. The container of claim 1 wherein the ethylene copolymer is of an acidterpolymer resin consisting of an interpolymer of ethylene, vinylacetate, and an organic acid.

1. A CONTAINER FOR COOKING FOOD INCLUDING: THE CONBINATION OF ADISH-LIKE RAY OF POLYETHYLENE TEREPHTHALATE, THE TRAY, HAVING A BOTTOMPART AND A BODY PART AND AN OUTWARDLY PROJECTING PERIPHERAL FLANGEHAVING AB UPPER SURFACE ON THE UPPER EXTERMITY OF THE BODY PART; ANADHESIVE OF AN ETHYLENE COPOLYMER AND PETROLEUM WAX HAVING A COMPOSITIONCONSISTING ESSENTIALLY OF (A) 30 PERCENT TO 40 PERCENT BY WEIGHT OFPETROLEUM WAX AND (B) 70 PERCENT TO 60 PERCENT BY WEIGHT OF SUH ETHYLENECOPOLYMER, POSITIONED ONLY ON THE UPPER SURFACE OF THE FLANGE, THEADHESIVE HAVING A MELT FLOW OF LESS THAN 0.2 GRAMS AT 320*F. (160*C.)FOR 10 MINUTES AND A SURFACE FREE ENERGY OF LESS THAN 30 DYNES; AND ASUBSTANTIALLY FLAT COVER PART OF POLYETHYLENE TERPHTHALATE HAVING LOWERSURFACE PARTS ADAPTED TO BE BONDED TO THE FLANGE BY THE ADHESIVE,WHEREIN SAID ADHESIVE IS FREE FROM FLOW AT COOKING TEMPERATURE UP TOABOUT 400*F AND SAID COMBINATION EXHIBITS A BOND STRENGTH GREATER THAN100 GRAMS/INCH AT 400*F AND LESS THAN 500 GRAMS/INCH AT ROOM TEMPERATUREWHEN SAID COVER PART IS BONDED TO SAID FLANGE.
 2. The container of claim1 wherein the polyethylene terephthalate in the tray has an intrinsicviscosity of at least 0.7.
 3. The container of claim 1 wherein theethylene copolymer is of an acid terpolymer resin consisting of aninterpolymer of ethylene, vinyl acetate, and an organic acid.